The present invention relates to electronic waveform filtering circuitry and, more particularly, to a novel capacitive commutating filter for detecting a particular waveform frequency in an input signal.
Many data processing applications require a filter for detecting the presence of a waveform at a particular frequency. Many forms of capacitive commutating filter are known for providing this frequency detection function. The prior art filters utilized passive or active circuits in which the filter frequency is fixedly established and is dependent upon the ratio of values of various resistive, capacitive and/or inductive components. Such component values must be established with relatively narrow tolerance and cause the filter to not only be relatively expensive to fabricate, but also restrict operation to only a single frequency. It is highly desirable to have a capacitive commutating filter which has a filter frequency capable of being precisely and repeatedly controlled by an external signal, and particularly one of digital nature. It is also advantageous to provide a capacitive commutating filter in which the filter frequency is relatively independent of process variation or temperature drift of the electronic components utilized in the circuit.